'I must live up to what people expect'

On stage he is the embodiment of operatic passion and fire. In real life Placido Domingo is the gentlest of men and driven by his art - to the world's greatest tenor, mañana is just another working day

'I have the big passion,' said Plácido Domingo. He beamed and spread his arms to enfold a global contingent of fans. In his endearingly unidiomatic English, he was declaring his passion for singing. 'It is for me still like the beginning of my career.'

That career began with an engagement in Mexico 46 years ago. After apprentice years in Israel, Domingo became a fixture at the Metropolitan Opera in New York, at Covent Garden (where he is appearing in Wagner's Die Walküre) and everywhere else, admired for his vocal ardour and reckless physical impetuosity. Laurence Olivier, after seeing him in Verdi's Otello in Vienna, ruefully remarked that Domingo acted it as well as he had done himself - 'and,' he added, 'the bloody fellow can sing as well!'

The tours of the Three Tenors, inaugurated at the World Cup in Rome in 1990, made Domingo a pop idol and turned the artist into a commercial concession. He advertised Rolex watches and shoe-horned himself into a white tuxedo to cavort with some feathered showgirls in a television commercial for a brand of carbonated Spanish wine.

Unable to be idle, he sought engagements as a conductor to occupy days when he was not singing, and hurtled between cities or continents to fulfil these commitments. Now he is a bi-coastal administrator, running opera companies in Los Angeles and Washington (and keeping them in business with his own funds: he personally made good on a donation of $2 million promised by the financier Alberto Vilar, who is awaiting trial for fraud, and did so in Vilar's name, to save the shamed benefactor's face). 'I accept the positions in LA and Washington,' he said, 'because I think I am not going to be singing so long. But ah, what can I do?' He clasped his unstoppably eloquent throat, unsure whether to caress it or to throttle himself.

Domingo is now 64. There are other hypotheses about his age, though when I referred to one of the alternatives a few years ago I received a politely huffy letter from his New York publicist, who enclosed smudged Xeroxes of his client's birth certificate and a document attesting to his baptism. Domingo himself, I suspect, is these days proud of his longevity rather than shifty about it.

'Yes,' he said, 'I have been so much time around - 34 years here, 38 years at the Met. This is a long chunk of life, it is a complete life of some people. When I began I was always the youngest in the cast. Now I am the oldest, the more experienced. But I cannot give less because of that. When I do not feel the excitement, then I will stop. Until then, what I must do is pass on my enthusiasm to the others who are younger and show how live theatre is supposed to be.'

He is, I can confirm, unstinting. When I arrived at Covent Garden, he was rehearsing onstage. The other cast members fish-mouthed to spare themselves, but Domingo sang with an exultantly full voice as he pulled swords from trees and made incestuous love to his sister, seeming to derive energy from the lava flow of the orchestra.

He engaged just as urgently with the gaggle of colleagues, helpers and hangers-on who trailed him to his dressing room. He may be named after a peaceful Sunday, but in person Domingo is more like a Monday morning rush hour that lasts all week.

Our conversation occupied what should have been his break between acts; I was confronted by a Wagnerian savage dressed in tattered pelts and covered with battle scars and purple gashes, who all the same played with a pair of reading glasses as he talked. Watching him emote onstage and schmooze in the backstage corridors had made me weary, so I asked what kept him going. 'It is my responsibility. The public does not know if you are tired from travelling or not well. I had a little cold the other day, from the air-conditioning. I cannot let that stop me. I must live up to what people expect.'

That sense of obligation, rare in performers (especially among the flighty narcissists who sing opera), is the essence of his character and accounts for the esteem in which he is held. He always justifies the price of the ticket, because he sets out to earn his reputation all over again every time he sings.

Tenors, of course, especially when brandishing swords as Domingo had been doing in Die Walküre, are emblems of virility; their voices are phallic symbols. The redoubtable Wagnerian soprano Birgit Nilsson was once asked what it was like to sing Isolde with an unappealing Tristan. 'Oh,' she replied, 'I just close my eyes and think of Plácido Domingo.'

She was not alone in indulging the fantasy. After a performance of Samson et Dalila in San Francisco, I remember watching Domingo fend off a scantily clad socialite who offered all her available limbs and begged 'Please sign me!' He chose her wrist as the safest option, scrawled his name on it with her felt pen and hastened towards his wife, Marta, described as 'beady' by Jeremy Isaacs.

Domingo calls Marta his inspiration and his guide; I suspect she is also his watchdog and superego. Last weekend, his peaceful Sunday was preceded by an agitated Saturday, when he read an offensive piece in the Times that rehashed ancient tittle-tattle about crushes and cited a book on the private lives of Domingo, Pavarotti and Carreras written by Monica Lewinsky's mother.

'You have to suffer words from people who do not know you,' sighed Domingo. 'They have the power of ink.' His phrase was touchingly true, despite its clumsiness: he knows that ink can be used to besmirch him. 'I cannot answer back. But sometimes I wish I could meet the ones who write these things, to ask them why.' A more effective deterrent for the scandal-mongers might be a rendezvous with the fearsome Marta Domingo.

We may expect tenors to be priapic fellows, but Domingo is strictly respectable, even prim. Hence his odd emphasis when I prodded him to nominate the operatic character to whom he feels closest. 'I love Cavaradossi in Tosca. He is an artist, he believes in human rights - and he is legitimately in love with the soprano!'

Legitimation matters to Domingo. He was happy to don the putty proboscis of the absurd, love-lorn hero in Franco Alfano's Cyrano, which he will sing next year at Covent Garden, because he approves of the man. 'Cyrano is so positive, so human. He copes with tragedy by mocking himself and he shows the others how to do the right thing.' He thought carefully when I asked him to choose the character he most dislikes. After discounting Puccini's cynical adventurers, he decided on Rasputin; he sang the part in Deborah Drattell's Nicholas and Alexandra, commissioned by his company in LA. 'He is a drunkard, a man who is filthy in his habits, and he uses his connections with power to have all these women - bah!'

I mentioned a recipe for weight loss that was once attributed to Domingo: 'Eat less, make love more.' 'No, no, no,' he insisted, mildly outraged. 'I never said such a thing! And it is not even true. If you make love too much, then you must eat a lot, to keep up your strength. I control the weight by discipline.'

The remark sums up his puritanism; he is a Mediterranean man with a very Nordic work ethic, which makes him impatient with the self-indulgence of Pavarotti. 'It is tough for Luciano. He was most of his life happy. But now with the hip and the legs so weak, it is difficult for him to move around. And the problems were provoked by himself. For all those years he just sat, he didn't exercise. This is what happens.'

Onstage, Pavarotti always looked blithe and benign (even when he was meant to be participating in a tragedy). Offstage, he is apparently a more troubled and darkly complex person. Domingo, I suggested, is the opposite. As a singer, he is best when most agonised: the epilepsy of Otello, the rabid jealousy of Don José in Carmen or Canio in Pagliacci, the wailing laments of Cavaradossi in the torture chamber. But the man you meet is amiable, gentle - yes, placid. So where do the performances come from? His answer hinted again at self-discipline, the careful conserving or repression of emotion, and implied that the theatre gives him a secret life, like that we enjoy in dreams.

'I keep everything for the stage, except happiness. That I want to have when I am not in the theatre - when I am with my family. The stage is the place for violence and anguish and big tragedies. It is good to pretend, it is wonderful if we can confine the disasters in this way. I am a happy man, but I must say that it is wonderful to suffer onstage, it is the most beautiful thing of all!'

This, put another way, is the Aristotelian doctrine of tragic catharsis: we are purged and exalted by watching someone else's mental distress and physical torment. Domingo is our designated sufferer. No wonder it's such a relief to see him recover from the evening's death, resurrected by our applause.

The voice is an organ embedded in the body, so opera singers like athletes rely on their youthful vitality. Domingo, having outlived that, now possesses a wisdom that is more precious. 'Of course at my age I cannot be Romeo. But I can play Wagner's Parsifal, who is maybe younger than Romeo at the start, because we see him maturing, and by the end - who knows? - he is perhaps an immortal.'

This journey through the ages of man will be placed on view when he sings the title role in Verdi's Simon Boccanegra at the Met. 'Perhaps I should retire as a tenor then come back to do this baritone part - just this one, it will not be another career! Boccanegra is so fatherly, that is why I love him. But the opera covers his whole life. At the start you are the pirate, and then 20 years later you appear again as the Doge'.

His use of the second person made clear his method of inhabiting a character: he was already imagining what it would be like to swagger piratically and feeling his way ahead - maturing, ripening, recognising mortality - towards the grave, regretful conscience of Boccanegra the elder statesman. 'After that,' he concluded, making the sign of the cross, 'I will probably say Amen.'

Before this leave-taking, at the Met in November 2006, he will appear in the world premiere of an opera written for him by Tan Dun, based on Bertolucci's film The Last Emperor. Discussing it, Domingo entered a significant proviso: 'It has a different title, it is to be called The First Emperor.'

The word 'last' is not yet in his vocabulary; he is unprepared for anything terminal. That may be the most invigorating thing about him. Performers live in the moment and remind us of its transitoriness and its irreplaceable value. This is Domingo's creed and it explains his refusal to whisper during the rehearsal of Die Walküre'. Pavarotti was once caught lip-synching to a recording at one of his concerts; such behaviour, for Domingo, would be blasphemous. 'What we do is live theatre. Everything is at stake for us and what we show to the public is how it is to be alive.'

After the rehearsal, he had a day of vocal rest before his opening at Covent Garden. I think he was dreading the enforced silence: not to sing or even speak must for him be tantamount to death. His remedy for the miseries of travel is just as jauntily existential. 'When I arrive in London, I never think how many hours different it is in New York, or wherever else I have come from. This is where I am now - this place, this time, here and now.'

Here and now, however, the music summoned him. Fresh scars were painted on to his beleaguered body, and with his wounds streaming, he valiantly bounded off for the second act of Die Walküre, ready to die again as if for the first time.

· Plácido Domingo is appearing in 'Die Walküre' at the Royal Opera House Covent Garden, London WC2 on 12 and 15 July, with a further performance at the Proms on 18 July